(Invited) Thermochemistry of Redox Active Oxides and Its Relevance to Solar Fuel Generation

Laboratories around the world are pursuing a variety of promising strategies for converting solar energy into a reliable energy source for on-demand utilization. We describe here a thermochemical approach for achieving this goal using solar heat as the energy source and redox active non-stoichiometric oxides as the reaction medium. Specifically, upon exposure to high temperatures and/or inert gas, the oxide undergoes reduction (without change in crystalline phase) to release oxygen. On cooling in the presence of H₂O (or CO₂), the oxide is reoxidized, releasing H₂ (or CO). We compare the behavior of a variety of oxides, including those of the fluorite structure-type (ceria and its derivatives) and those of the perovskite structure-type (La_1-xSr_xMnO₃). A shared characteristic of the most promising materials is that bulk oxygen diffusion (chemical diffusion) is fast such that fuel production rates are limited either by surface reaction kinetics or gas-phase mass transfer rates.